1. Field
The present application relates to a crystallization method of a silicon thin film, and more particularly, to a mask for laser crystallization and a crystallization method using the mask.
2. Discussion of the Related Art
Flat panel display (FPD) devices having high portability and low power consumption have been the subject of recent research and development. Among various types of FPD devices, liquid crystal display (LCD) devices are commonly used as monitors for notebook and desktop computers because of their ability to display high-resolution images, wide ranges of different colors, and moving images.
In general, an LCD device includes a color filter substrate and an array substrate separated from each other by a liquid crystal layer. The color filter substrate and the array substrate include a common electrode and a pixel electrode, respectively. When a voltage is supplied to the common electrode and the pixel electrode, an electric field is generated that affects orientation of liquid crystal molecules of the liquid crystal layer due to optical anisotropy within the liquid crystal layer. Consequently, light transmittance characteristics of the liquid crystal layer become modulated and images are displayed by the LCD device.
Active matrix type display devices are commonly used because of their superior display of moving images. Active matrix-type display devices include pixel regions that are disposed in matrix and a switching element, such as a thin film transistor (TFT), is formed at each pixel region. Recently, LCD devices that include TFTs using polycrystalline silicon (p-Si) have been widely researched and developed. In an LCD device using polycrystalline silicon, specifically, both a display region TFT and a driving circuit may be formed on one substrate. Moreover, since an additional process of connecting the TFT of the display region and the driving circuit is not necessary, the total fabrication process for the LCD device is simplified. Since the field effect mobility of polycrystalline silicon is several-hundred times as great as that of amorphous silicon, the LCD device using polycrystalline silicon has a short response time and high stability against heat and light.
FIG. 1 is a schematic view showing a liquid crystal display device according to the related art where a display TFT and a driving circuit are formed on one substrate. In FIG. 1, a driving circuit portion 3 and a pixel portion 4 are defined on a single substrate 2. The pixel portion 4 is disposed at a central portion of the substrate 2, while the driving portion 3 is disposed at left portion and top portions of the pixel portion 4. The driving circuit portion includes a gate driving circuit 3a and a data driving circuit 3b. The pixel portion 4 includes a plurality of gate lines 6 connected to the gate driving circuit 3a and a plurality of data lines 8 connected to the data driving circuit 3b. The gate line 6 and the data line 8 cross each other to define a pixel region and a pixel electrode 10 is formed in the pixel region. A thin film transistor (TFT) “T” as a switching element is connected to the pixel electrode 10. The gate driving circuit 3a supplies a scan signal to the TFT “T” through the gate line 6 and the data driving circuit 3b supplies a data signal to the pixel electrode 10 through the data line 8. The gate driving circuit 3a and the data driving circuit 3b are connected to an input terminal 12 of external signals. Accordingly, the gate driving circuit 3a and the data driving circuit 3b process the externals signals from the input terminal 12 to generate the scan signal and the data signal.
Even though not shown in FIG. 1, the TFT includes a semiconductor layer, a gate electrode, a source electrode and a drain electrode, and the semiconductor layer uses polycrystalline silicon because of higher field effect mobility.
Polycrystalline silicon may be formed by crystallizing amorphous silicon. A laser annealing method, where a laser beam is irradiated onto an amorphous silicon film, finds wide use as a crystallization method. However, since the surface temperature of the irradiated amorphous silicon film reaches about 1400° C., the top surface of the silicon film is apt to oxidize. Particularly, since the laser beam irradiates the silicon film several times during the laser annealing method, silicon oxide (SiO2) may be created on the top surface of the silicon film when the irradiation of the laser beam is performed in ambient air. Accordingly, the laser beam may be irradiated under a vacuum of about 10−7 to 10−6 Torr. To solve the above problems of the laser annealing method, a sequential lateral solidification (SLS) method using a laser beam has been suggested and researched.
The SLS method utilizes the phenomenon that grains of a silicon film grow along a direction perpendicular to a border surface of a liquid phase region and a solid phase region of the silicon film. In the SLS method, grains grow along one lateral direction by adjusting an energy density and an irradiation range of a laser beam and moving the laser beam (Robert S. Sposilli, M. A. Crowder, and James S. Im, Material Research Society Symp. Proc. Vol. 452, pages 956-957, 1997). Since the grain size of a silicon film is enlarged by the SLS method, a TFT having a channel region of single crystalline silicon may be obtained.
FIG. 2 is a schematic plane view showing a mask for laser crystallization according to the related art. In a SLS method, a mask having slits is disposed over an amorphous silicon layer and the amorphous silicon layer is crystallized by repetition of irradiation of a laser beam and movement of the mask along two directions. In FIG. 2, a unit area of an amorphous silicon layer may be irradiated twice and a mask 16 for laser crystallization may be referred to as a two-shot type. The mask 16 has a crystallization pattern 14 including a plurality of slits 13, each of which has a width of several micrometers. The crystallization pattern 14 has first and second blocks “IIa” and “IIb.” The slits 13 in the first block “IIa” alternate with the slits 13 in the second block “IIb.” When an amorphous silicon layer is crystallized, the mask 16 may move by a distance corresponding to a width of each block “IIa” or “IIb.” Accordingly, a unit area of the amorphous silicon layer is crystallized by twice irradiation of a laser beam. A size of the laser beam irradiated onto the mask 16 corresponds a first region “IIIc,” where the laser beam has a uniform energy density.
FIG. 3A is a schematic plane view showing a size of a laser beam for laser crystallization according to the related art, FIG. 3B is a schematic cross-sectional view, which is taken along a line “IIIa-IIIa” of FIG. 3A, showing a profile of a laser beam for a laser crystallization according to the related art and FIG. 3C is a schematic cross-sectional view, which is taken along a line “IIIb-IIIb” of FIG. 3A, showing a profile of a laser beam for laser crystallization according to the related art. In FIG. 3A, a laser beam 20 includes a first region “IIIc” having a first energy density corresponding to a complete melting regime at a central portion and a second region “IIId” having a second energy density lower than the first energy density at a peripheral portion.
In FIGS. 3B and 3C, each of transverse and longitudinal profiles of a laser beam 20 includes a first region “IIIc” and a second region “IIId.” The first region “IIIc” may be referred to as a top hat region. A first energy density of the first region “IIIc” corresponds to a complete melting regime and a second energy density of the second region “IIId” corresponds to a partial melting regime. Since a laser beam having an energy density corresponding to a complete melting regime is irradiated in a SLS method, a third region “IIIe” corresponding to a crystallization pattern 14 (of FIG. 2) should be disposed inside the first region “IIIc” for laser crystallization. The third region “IIIe” may be designed smaller than the first region “IIIc” on the basis of alignment margin.
However, when the mask for laser crystallization is changed, the mask may be misaligned over the alignment margin. Specifically, the misalignment of the mask may become more serious for a long rest time period between laser annealing processes or a long time period of a laser annealing process.
FIGS. 4A and 4B are a schematic plane view and a schematic cross-sectional view, respectively, showing misalignment of a mask for laser crystallization according to the related art. In FIGS. 4A and 4B, a mask 16 for laser crystallization is misaligned with a laser beam such that a first region “IIIc” of the laser beam does not cover a crystallization pattern 14. Accordingly, a laser beam corresponding to a slope region “IV” having a third energy density lower than a first energy density of the first region “IIIc” may be irradiated onto an amorphous silicon layer, thereby deteriorating crystallinity of a resultant polycrystalline silicon layer.
An LCD device where a driving circuit and a pixel TFT are formed on a single substrate may be fabricated using a multi-pattern mask for laser crystallization. Since the requirements of a driving TFT in the driving circuit is different from that of the pixel TFT, a semiconductor layer of the driving TFT may be crystallized using a crystallization pattern different from that for a semiconductor layer of the pixel TFT. In order to simplify the process and reduce the production cost, a multi-pattern mask for laser crystallization having several different crystallization patterns may be used for an LCD device having a driving circuit and a pixel TFT on a single substrate. Since the multi-pattern mask is aligned with the amorphous silicon layer more frequently, the misalignment may cause more serious problems in crystallinity.